a) Field of the Invention
The invention relates to a tool for coating an optical disc, a method for coating an optical disc, and a method for fabricating an optical disc. More particularly, the invention relates to a tool for coating an optical disc, a method for coating an optical disc, and a method for fabricating an optical disc that are adaptable to forming a thick annular coating layer with uniform thickness defined within a desired coating area when using a highly viscous coating material.
b) Description of Related Art
Coating an optical disc to form a flat coating layer thereon is an important step in fabricating an optical disc. Conventional coating methods include sputtering, spin-coating, printing, chemical vapor deposition and so on. However, each of the conventional methods has limitations on its utilization.
Spin-coating is a process in which an optical disc substrate is rotated at a higher rotation speed of 100 rpm to 8000 rpm to spread out and uniformly distribute coating material on the optical disc substrate. The advantage of such method is that a coating layer with uniform thickness can be obtained. Although a thickness more than 100 μm can be formed by spin-coating in special processes, a coating layer formed with viscous coating material may have uneven thickness or severe water ripples because viscous coating material cannot be spread evenly on the optical disc substrate by spin-coating. When the viscosity of coating material is greater than 10000 cps, such coating material cannot be spun so as to be spread. Also, since the substrate to be coated must be rotated at a high rotation speed between 100 rpm and 8000 rpm to perform spin-coating, spin-coating cannot be used to form a coating layer on a substrate that is unsuitable for high-speed rotation. In addition, when performing spin-coating, a part of the coating material is spun over and out of the optical disc substrate region, thus causing a waste of coating material or a problem where waste material must be recycled.
As for sputtering, printing, and chemical vapor deposition, although viscous coating material may be used, a coating layer with thickness greater than 0.1 mm is difficult to form, or the flatness of the formed coating layer with high thickness is poor. For instance, when performing vacuum sputtering, only a coating layer with thickness less than 10 μm can be formed. Common printing methods like relief printing, intaglio printing, and offset printing can form a coating layer with thickness less than 20 μm only. Screen printing can form a coating layer with thickness of approximately 100 μm, but the usage of this method is limited to an object that has no protruding portion. For example, if an optical disc substrate that is to be coated has a thick supporting portion at the central part of the substrate, making the central supporting portion to be protruded, then the method of screen printing cannot be employed to form a coating layer. Moreover, besides printing methods, most of the remaining methods would have difficulty in defining a coating area, like forming an annular coating layer with predetermined width on an annular substrate. Furthermore, scraping technique is in general applied in coating the entirety of larger area, so excess coating material is likely to be scraped off in operation, and thus a waste of coating material is inevitable and limiting the coating area, like forming an annular coating layer, is difficult.
It is not easy to find an appropriate coating method to form, on an optical disc substrate, a uniform annular coating layer with large thickness by using coating material having higher viscosity while defining the coating area to save cost of coating material.